The project objective is to learn more about the endocrine regulation of cellular gonadotropin receptor populations using either in vitro or in vivo models. Studies during the past year have established that it is possible to achieve induction of functional luteinizing hormone (LH) receptors utilizing the proper ovarian cell type (pig or rat) in the presence of insulin and follicle stimulating hormone. Autoradiographic studies showed that the cellular pattern of induction was non-uniform, a finding which had significant implications with regard to the calculation of receptor numbers of a "per cell" basis. Induction is inhibited significantly by growth factors such as epidermal growth factor; these findings may be important in working out the biochemical basis of follicular recruitment and/or atresia. Another in vitro culture system, i.e., organ culture of ovarian theca interna, was characterized with regard to steroid (estrogen and progesterone) and non-steroid production. An unidentified non-steroidal factor, produced by such cultures, is capable of preventing to a significant degree the loss of granulosa cell LH receptors normally seen under control conditions. Future studies will be directed toward a more detailed analysis of receptor regulation under culture conditions.